Transesterification of a simple ester compound with a simple alcohol compound is known to occur under basic conditions. The transesterification reaction is an equilibrium reaction which can be driven to completion by removing the alcohol moiety evolving from the cleaved ester. If the cleaved alcohol moiety is a low molecular weight lower alkyl alcohol such as methanol or ethanol, removal by evaporation is quite easy. It has been found that transesterification as a curing mechanism for crosslinking polymers used in paint coatings provides an attractive cure mechanism for producing thermosetting protective coatings since cleaved lower alkyl alcohols can be easily removed from the coating by simple air dry evaporation thereby driving the transesterification reaction to completion. Protective surface coatings commonly known as paint coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as a fluid vehicle for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Conventional thermosetting polymers often require high temperatures as well as external crosslinkers. Some crosslinkers, such as melamines in conventional industrial coatings or triglycidyl isocyanate for powder coatings, can cause toxicity problems. Ambient cure technologies such as isocyanates cause toxicity problems. Also, the release of volatile by-products, such as caprolactam, from some of these materials can cause film defects, such as cratering and bubbling.
Several related patents and pending patent applications pertaining to non-aqueous coatings based on hydroxyl functional polymers adapted to cure by transesterification with a carboxylic ester functional polymer in the presence of transesterification catalysts are as follows: U.S. Pat. No. 4,749,728 where the catalyst comprises an onium salt or Lewis base in combination with a catalytic epoxy compound; U.S. Pat. No. 4,906,693 where the catalyst comprises certain inorganic salts in combination with a catalytic epoxy compound; U.S. Pat. No. 4,897,450 based on transesterification of a beta hydroxyl ester addition polymer with a hydroxyl functional polymer; U.S. Ser. No. 138,149 filed Dec. 28, 1987 where the catalyst comprises a conjugate base of a weak acid having a pKa between 2.5 and 1.4 in combination with a catalytic epoxy compound; and U.S. Ser. No. 251,762 filed Oct. 3, 1988 based on certain polyester polymers.
With respect to prior art, several patents disclose the use of acids, bases, metal salts, and organic metal complexes as catalysts for transesterfying polymers such as U.S. Pat. Nos. 4,362,847; 4,376,848; 4,332,711; and 4,459,393 wherein octoates or naphthenates or lead, zinc, calcium, barium, and iron are disclosed as transesterification catalysts. Other prior art patents are as follows.
Green U.S. Pat. No. 4,559,180 discloses an organic syntheses process for simple transesterification of very low molecular weight simple carboxyl ester compounds with similar simple alcohols by reacting the same in the presence of an epoxide and a Lewis base (containing a Group V element) or a cyclic amidine. The Green patent does not pertain to coatings or polymers but merely discloses simple chemical reactions between very low molecular weight chemical compounds.
Dante and Parry have shown that phosphonium halides, such as ethyltriphenyl phosphonium iodide, are efficient catalysts for (a) 1,2-epoxide reactions with phenols to yield hydroxyl ethers (U.S. Pat. No. 3,477,990), and (b) polyepoxide reactions with carboxylic acids or acid anhydrides (U.S. Pat. No. 3,547,885). The patents suggest that polyepoxides and phenols can be reacted to form phenolic hydroxyl ethers with phosphonium salts as catalysts. The counterion of the phosphonium moiety is the anion portion of a carboxylic acid, or acid ester, such as in ethytriphenyl phosphonium acetate (U.S. Pat. No. 3,948,855).
Barnhoorn et al (U.S. Pat. No. 4,459,393) teach self-crosslinking thermosetting resin compositions obtained from the reaction of a beta-hydroxyalkyl ester of an alpha, beta-carboxylic acid with a primary mono- or polyamine to give a product have 1 to 2 amino hydrogens and further reacted with a polyglycidyl ether of a polyhydric phenol so that the final resin adduct has more than one beta-hydroxyalkyl ester group and amine groups having 1 to 2 amine hydrogen atoms per molecule. Transesterification catalysts known in the art are taught.
Subramanyam et al (U.S. Pat. No. 4,376,848) teach the preparation of water dilutable electrocoating compositions having tertiary amino-containing basic binders by reacting a secondary amino group compound with an olefinically double-bonded epoxy and the copolymerization of this product with at least one ethylenically bonded polymerizable monomer wherein said binders can self-cure and be cured in combination with amine resins and/or phenolic resins. Common transesterification catalysts are taught.
Velko patents disclose resinous compositions curable through a transesterification curing mechanism based on conventional heavy metal catalysts. For instance, U.S. Pat. No. 4,423,167 discloses a polymeric polyol adapted to be crosslinked with a polyester having at least two beta-alkoxyester groups in the presence of conventional transesterification catalysts. Similarly, U.S. Pat. No. 4,489,182 is based on a crosslinking agent having at least two delta-hydroxy ester groups, while U.S. Pat. No. 4,423,169 is based on a crosslinking agent having at least two beta- and/or gamma-ester groups, and U.S. Pat. No. 4,423,168 is based on a crosslinking agent having at least two beta-amide ester groups. The Velko patents utilize as catalysts organic salts of conventional heavy metal catalysts such as lead, zinc, iron, tin and manganese.
It now has been found that emulsion polymers produced by emulsion copolymerization of ethylenic monomers in water and contain both carboxyl ester and hydroxy polymer chains can be cured by transesterification at ambient or low bake temperatures. The coreaction emulsion polymer provides an excellent binder for paint coatings exhibiting good cure and film properties under ambient and low bake conditions. This discovery provides low toxicity (non-formaldehyde), very low VOC, good cured films exhibiting solvent resistance, hardness, flexibility, and &gt;160 inch pounds of impact resistance (forward and reverse). Ambient and 140.degree. F. cure can be obtained with a two pack system. One pack stable cure is obtained at 250.degree. to 300.degree. F. with proper choice of catalyst.
Industrial latex coatings offer significant advantages over solvent borne and most aqueous dispersed coatings, in that VOC is exceptionally low. Latex polymerization produces high molecular weight polymer, which provides better physical and chemical properties than medium and low molecular weight polymers. Crosslinking chemistries formerly available for latex use, however, are limited, and the most commonly used crosslinkers (i.e. N-methylolacrylamide) are undesirable formaldehyde based. Hence, this discovery satisfies an ongoing need for new, low bake, low toxicity crosslinking for latex systems. Room temperature cured coating films exhibited above 90% gloss (60.degree.), more than 200 MEK rubs solvent resistance, H pencil hardness without the need for coalescent solvents, good adhesion to polycarbonates, sheet molding compounds, ABS, and similar plastic substrates, and cured films resist yellowing. The unanticipated finding in this invention was that excellent ambient cure is obtainable under certain conditions in the presence of transesterification catalysts, particularly when a blend of epoxies (bisphenol epoxies and glycidyl acrylate or methacrylate) are used as a cocatalyst with a tertiary amine.
These and other advantages of this invention will become more apparent by referring to the detailed description and the drawings.